Exhaust emission control device for an internal combustion engine

ABSTRACT

A three-way catalyst ( 30 ) comprises a microporous catalyst element ( 30   a ) having a micropore group whose average pore opening size is smaller than molecular size of HC in a washcoat and a macroporous catalyst element ( 30   b ) having a macropore group whose average pore opening size is larger than the molecular size of HC in a washcoat.

CROSS-REFERENCE TO THE RELATED ART

This application incorporates by reference the subject matter of Application No. 2004-2999, filed in Japan on Jan. 8, 2004, on which a priority claim is based under 35 U.S.C S119(a).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust emission control device for an internal combustion engine,. and more specifically to a technology for raising purification efficiency of a three-way catalyst.

2. Description of the Related Art

A three-way catalyst is commonly used as an exhaust gas purifying catalyst for an internal combustion engine for a vehicle. The three-way catalyst is designed to bring an exhaust air-fuel ratio close to a theoretical air-fuel ratio (Stoichio) for optimization of the oxidation of HC (hydrocarbons) and CO (carbon monoxide) and the reduction of NOx to promote the purification of exhaust gases.

An exhaust emission control device that has been lately developed has a construction in which the catalyst is for example a porous structure to trap NOx, Oxygen (O₂), HC and CO in pores, to thereby trap HC and CO in the pores and oxidize them using the trapped NOx and O₂ in a reducing atmosphere, and on the other hand to thereby trap NOx and O₂ in the pores and reduce NOx using the trapped HC and CO in an oxidizing atmosphere.

Moreover, a technology for encouraging only reactions useful for the purification of NOx by reducing the pores in size to prevent HC, which serves as a reducing agent, from approaching the oxidation catalyst in the porous structure has also been developed (see Unexamined Japanese Patent Publication No. 2001-525241 as an example).

It is noted that, generally in the three-way catalyst, the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed. This means that if it is possible to separate HC and CO from each other and to preferentially cause the oxidation-reduction reaction of CO and NOx, NOx can be improved in its purifying performance.

In the reducing atmosphere, however, HC and CO are mixed in the exhaust emission. Conventional technologies related to porous structures, including the technology disclosed in the above-mentioned publication, are not designed to trap HC and CO separately from each other. This causes the problem that the presence of HC having large molecular size hinders the oxidation-reduction reaction of CO and NOx having small molecular size, thereby decelerating the oxidation-reduction reaction of CO and NOx which are fast in reaction speed. Such deceleration in the oxidation-reduction reaction of CO and NOx generates the problem that part of CO is reacted with O₂, resulting in a shortage of O₂ for the oxidation of HC.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and an object thereof is to provide an exhaust emission control device for an internal combustion engine designed to actively separate HC and CO and preferentially produce oxidation-reduction reaction of CO and NOx on a catalyst, to thereby upgrade an exhaust gas purifying performance.

To achieve this object, in the exhaust emission control device for an internal combustion engine according to the present invention, there is provided a three-way catalyst in an exhaust channel of an internal combustion engine. The three-way catalyst comprises one or more catalyst elements and has two or more pore groups different in average pore opening size in a washcoat.

A further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific example, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein:

FIG. 1 is a schematic view of a construction of an exhaust emission control device for an internal combustion engine according to an embodiment 1 of the present invention, which is installed in a vehicle;

FIG. 2 shows a view (a) of a quarter portion of a unit cell of a microporous catalyst element, an enlarged view (b) of catalysts coating the quarter portion, and an enlarged view (c) of one particle of a washcoat (W/C);

FIG. 3 shows a view (a) of a quarter portion of a unit cell of a macroporous catalyst element; an enlarged view (b) of catalysts coating the quarter portion; and an enlarged view (c) of one particle of a washcoat (W/C);

FIG. 4 is a graph showing frequency distribution of pore opening size in the microporous catalyst element (solid line) and in the macroporous catalyst element (broken line), and average pore opening size X and average pore opening size Y of the microporous and macroporous catalyst elements, respectively;

FIG. 5 is a flowchart showing a control routine of O₂ F/B control according to the first embodiment;

FIG. 6 is a view showing a three-way catalyst according to another embodiment of the first embodiment;

FIG. 7 is a view showing a three-way catalyst according to a second embodiment;

FIG. 8 is a view showing a three-way catalyst according to another embodiment of the second embodiment;

FIG. 9 is a view showing a quarter portion of a unit cell of a three-way catalyst according to a third embodiment;

FIG. 10 is a view showing a quarter portion of a unit cell of a three-way catalyst according to a fourth embodiment; and

FIG. 11 is a flowchart showing a control routine of A/F modulation control according to a fifth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained below with reference to the attached drawings.

Firstly, a first embodiment will be described.

FIG. 1 is a schematic view of a construction of an exhaust emission control for an internal combustion engine according to the present invention, which is installed in a vehicle. Hereinafter the explanation about the construction of the exhaust emission control device will be provided.

As illustrated in FIG. 1, in a cylinder head 2 of an engine body (such as a gasoline engine, and hereinafter simply referred to as engine) 1 that is an internal combustion engine, there is disposed an ignition plug 4 in each cylinder. Connected to the ignition plug 4 is an ignition coil 8 for outputting high voltage.

In the cylinder 2, an intake port is formed for each cylinder, and one end of an intake manifold 10 is connected to the cylinder 2 so as to communicate with each intake port. An electromagnetic fuel injection valve 6 is attached to the intake manifold 10, and a fuel supply device, not shown, having a fuel tank is connected to the fuel injection valve 6 through a fuel pipe 7.

An electromagnetic throttle valve 14 for adjusting an intake air amount is disposed upstream from the fuel injection valve 6 of the intake manifold 10 together with a throttle position sensor (TPS) 16 for detecting the opening of the throttle valve 14. Interposed in the upstream side of the throttle valve 14 is an air flow sensor 18 for measuring the intake air amount.

Also in the cylinder head 2, an exhaust port is formed for each cylinder, and one end of an exhaust manifold 12 is connected to the cylinder head 2 so as to communicate with the exhaust port.

An exhaust pipe (exhaust channel) 20 is connected to the other end of the exhaust manifold 12. In the exhaust pipe 20, a monolith-type three-way catalyst 30 having a catalyst support honeycombed in section is interposed as an exhaust gas purifying catalyst device.

The three-way catalyst 30 includes any one of copper (Cu), cobalt (Co), silver (Ag), platinum (Pt), rhodium (Rh) and palladium (Pd) as active metal for a washcoat of a support surface.

The three-way catalyst 30 not only has the active metal but includes a great number of pores formed in a washcoat. Specifically, the three-way catalyst 30 comprises a microporous catalyst element 30 a having a micropore group whose average pore opening size is smaller than molecular size (prescribed size) of HC and a macroporous catalyst element 30 b having a macropore group whose average pore opening size is larger than the molecular size of HC, in which the microporous catalyst element 30 a is disposed on the upstream side of the exhaust flow, and the macroporous catalyst element 30 b is disposed on the downstream side of the exhaust flow in series with the microporous catalyst element 30 a.

FIG. 2 shows a view (a) of a quarter portion of a unit cell of a microporous catalyst element 30 a together with an enlarged view (b) of catalysts coating the quarter portion and an enlarged view (c) of one particle of a washcoat (W/C). As illustrated in FIG. 2, in the microporous catalyst element 30 a, a large number of micropores S of smaller opening size than the molecular size of HC are formed in the washcoat.

FIG. 3 shows a view (a) of a quarter portion of a unit cell of a macroporous catalyst element 30 b together with an enlarged view (b) of catalysts coating the quarter portion and an enlarged view (c) of one particle of a washcoat (W/C). As illustrated in FIG. 3, in the macroporous catalyst element 30 b, a great number of macropores L of larger opening size than the molecular size of HC are formed in the washcoat.

FIG. 4 shows frequency distribution of pore opening size in the microporous catalyst element 30 a (solid line) and in the macroporous catalyst element 30 b (broken line), and average pore opening size X and average pore opening size Y of the microporous and macroporous catalyst elements, respectively. In this way, the microporous catalyst element 30 a and the macroporous catalyst element 30 b are differentiated in the average pore opening size. Therefore, the three-way catalyst 30 is capable of trapping CO, O₂, NOx and H₂ having smaller molecular size than HC in the micropores S in the microporous catalyst element 30 a located on the upstream side of the exhaust flow and is also capable of trapping HC having large molecular size in the macropores L in the macroporous catalyst element 30 b located on the downstream side of the exhaust flow.

For example, the pore opening size is controlled by an impregnation method, a CVD (Chemical Vapor Deposition) method, or the like.

The microporous catalyst element 30 a is for example zeolite 3A, Ca-mordenite, or the like, and about 3 to about 3.8 angstroms in diameter. On the other hand, the macroporous catalyst element 30 b is for example zeolite 5A, ZSM-5, β, or the like, and about 5 to about 6 angstroms in diameter. Additionally, the macroporous catalyst element 30 b may be an ordinary catalyst (such as one composed mainly of Al₂O₃ and the like), instead of being the above-described one.

Substances, on which control of effective pore diameters is introduced, include zeolite, SAPO (silicoaluminophosphate), and ALPO (aluminophosphate), but are not limited to these. Any other substances may be utilized as long as the substances are different in pore diameter. Moreover, the substances may be of any other size and shapes on condition that the substances can sieve HC, CO, NOx, H₂, etc.

On the upstream side of the three-way catalyst 30 of the exhaust pipe 20, there is disposed an air-fuel ratio sensor 22 for detecting an exhaust air-fuel ratio (exhaust A/F), based on the concentration of oxygen in the exhaust emission. Utilized as the air-fuel ratio sensor 22 is an O₂ sensor, but a linear A/F sensor (LAFS) or the like may be employed instead.

An ECU (electrical control unit) 40 comprises an input/output device, memories (ROM, RAM, nonvolatile RAM, etc.), a central processing unit (CPU), a time counter, and the like. The ECU 40 performs comprehensive control of the exhaust emission control device including the engine 1.

Connected to an input side of the ECU 40 are various sensors, such as a crank angle sensor 42 for detecting a crank angle of the engine 1, in addition to the TPS 16, the air flow sensor 18 and the air-fuel ratio sensor 22. Detection information from these sensors is inputted to the ECU 40. Based on the crank angle information from the crank angle sensor 42, engine revolution speed Ne is detected.

Connected to an output side of the ECU 40 are various output devices, including the fuel injection valve 6, the ignition coil 8, the throttle valve 14 and the like. A fuel injection amount, fuel injection timing, ignition timing, and the like, which are calculated based on the detection information from the various sensors, are outputted to the various output devices.

Specifically, the air-fuel ratio is set to a proper target air-fuel ratio (target A/F), based on the detection information from the various sensors, and fuel of an amount adjusted according to the target A/F is injected from the fuel injection valve 6 with the right timing. Furthermore, the throttle valve 14 is adjusted to have proper opening, and spark ignition is carried out by the ignition plug 4 with the right timing.

More specifically, O₂ feedback (O₂ F/B) control is performed so that the exhaust A/F becomes the target A/F (for example, Stoichio), based on the information from the air-fuel ratio sensor 22. In response thereto, the fuel injection amount fluctuates, and practically, the exhaust A/F periodically fluctuates between a rich air-fuel ratio (rich A/F) side and a lean air-fuel ratio (lean A/F) side with the target A/F therebetween (air-fuel ratio modulating means).

Hereinafter, operation of the exhaust emission control device according to the present invention, which is thus constructed, will be described.

FIG. 5 shows a control routine of the O₂ F/B control in a flowchart, and explanations will be provided with reference to the flowchart.

First, Step S10 judges whether the exhaust A/F is the lean A/F or the rich A/F at present, based on the information from the O₂ sensor that is the air-fuel ratio sensor 22. If it is judged that the exhaust A/F is the lean A/F, rich operation is carried out in Step S12. To be concrete, the fuel injection amount is compensated to increase.

If the rich operation is carried out in the above manner, the exhaust A/F turns to the rich A/F, and a good deal of CO, in addition to HC, is contained in the exhaust emission. As a result, the reducing atmosphere is produced in the three-way catalyst 30.

As mentioned below, right before the reducing atmosphere is caused in the three-way catalyst 30, NOx and O₂ having smaller molecular size than HC are trapped in the micropores S of the microporous catalyst element 30 a. Therefore, when the reducing atmosphere is created in the three-way catalyst 30, the NOx and the O₂ are released to produce the oxidation-reduction reaction with CO and HC contained in the exhaust emission. Since the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed, the released NOx is reacted with CO by priority, whereas the released O₂ is well reacted with HC.

Once NOx and O₂ are sufficiently released, CO and H₂ having smaller molecular size than HC are well trapped in the micropores S of the microporous catalyst element 30 a located on the upstream side of the exhaust flow, while HC having large molecular size is successfully trapped in the macropores L of the macroporous catalyst element 30 b located on the downstream side of the exhaust flow. That is to say, in the three-way catalyst 30, CO and HC are actively separated and trapped in the microporous catalyst element 30 a and the macroporous catalyst element 30 b, respectively.

Thereafter if Step S10 judges that the exhaust A/F is the rich A/F, the lean operation is then provided in Step S14. To be concrete, the fuel injection amount is compensated to decrease.

Once the lean operation is implemented, the exhaust A/F turns to the lean A/F, and a great deal of NOx is contained in the exhaust emission together with O₂. The oxidizing atmosphere is therefore produced in the three-way catalyst 30.

When the oxidizing atmosphere is generated in the three-way catalyst 30, the CO, H₂ and HC trapped as described are released to cause the oxidation-reduction reaction with O₂ and NOx contained in the exhaust emission. In this case, as stated above, CO and HC are trapped in the microporous catalyst element 30 a and the macroporous catalyst element 30 b, respectively, so as to be separated from each other, and CO and H₂ are released in the microporous catalyst element 30 a located on the upstream side of the exhaust flow. As a result, the released CO is preferentially and reliably reacted with NOx contained in the exhaust emission in the microporous catalyst element 30 a located on the upstream side of the exhaust flow due in part to the fact that the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed. By using CO for reaction with NOx, the released HC is fully reacted with O₂ contained in the exhaust emission in the macroporous catalyst element 30 b located on the downstream side of the exhaust flow.

In other words, in the exhaust emission control device according to the first embodiment, the exhaust A/F is modulated between the lean A/F and the rich A/F due to the O₂ F/B control, to thereby satisfactorily create the oxidizing atmosphere and the reducing atmosphere. Therefore, CO and HC are repeatedly trapped well in the three-way catalyst 30 so as to be separated from each other in the reducing atmosphere. In the oxidizing atmosphere, on the other hand, the released CO and H₂ are preferentially and surely reacted with NOx contained in the exhaust emission without being hampered by the released HC. As a result, the purifying performance of NOx is upgraded. Furthermore, due to the use of CO and H₂ for reaction with NOx, the released HC is well reacted with O₂ contained in the exhaust emission, which enhances the purifying performance of HC. Consequently, the exhaust gas purifying performance of the three-way catalyst 30 is improved as a whole and is maintained in a high level.

The above explanation has been provided taking as an example the three-way catalyst 30 in which the microporous catalyst element 30 a and the macroporous catalyst element 30 b are completely coupled and integrated with each other in a direction of the exhaust flow as illustrated in FIG. 1. The microporous catalyst element 30 a and the macroporous catalyst element 30 b, however, do not have to be coupled to each other but may be located away from each other in the direction of the exhaust flow as another embodiment, as illustrated in FIG. 6.

Hereinafter a second embodiment will be described.

The second embodiment is different from the first only in that a three-way catalyst 301 is employed in place of the three-way catalyst 30.

As illustrated in FIG. 7, the three-way catalyst 301 comprises a macroporous catalyst element 301 a having pores whose average pore opening size is larger than the molecular size of HC and a microporous catalyst element 301 b having pores whose average pore opening size is smaller than the molecular size of HC. The macroporous catalyst element 301 a is disposed on the upstream side of the exhaust flow, and the microporous catalyst element 301 b on the downstream side. In other words, in the three-way catalyst 301, the microporous and macroporous catalyst elements are reversely positioned, compared to the three-way catalyst 30.

Operation of the exhaust emission control device, in which the macroporous catalyst element 301 a is disposed on the upstream side of the exhaust flow, and the microporous catalyst element 301 b on the downstream side as mentioned above, will be described below.

When the rich operation is carried out in the O₂ F/B control, and the reducing atmosphere is generated in the three-way catalyst 301, the trapped NOx and O₂ are released to produce the oxidation-reduction reaction with CO and HC contained in the exhaust emission as seen in the above embodiment. Since the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed, the released NOx is reacted with CO by priority, whereas the released O₂ is well reacted with HC.

Once NOx and O₂ are sufficiently released, HC of large molecular size is satisfactorily trapped in macropores L of the macroporous catalyst element 301 a located on the upstream side of the exhaust flow, while CO and H₂ having smaller molecular size than HC are satisfactorily trapped in micropores S of the microporous catalyst element 301 b located on the downstream side of the exhaust flow. Similarly to the above embodiment, in the three-way catalyst 301, HC and CO are actively separated from each other and trapped in the macroporous catalyst element 301 a and the microporous catalyst element 301 b, respectively.

On the other hand, when the lean operation is carried out, and the oxidizing atmosphere is created in the three-way catalyst 301, the trapped HC, CO and H₂ are released to cause the oxidation-reduction reaction with O₂ and NOx contained in the exhaust emission. In this case, HC and CO are separately trapped in the macroporous catalyst element 301 a and the microporous catalyst element 301 b, respectively. Therefore, in the macroporous catalyst element 301 a located on the upstream side of the exhaust flow, the released HC is well reacted with O₂ contained in the exhaust emission. At the same time in the microporous catalyst element 301 b located on the downstream side of the exhaust flow, the released CO and H₂ are well reacted with NOx contained in the exhaust emission without being hindered by the released HC.

This improves not only the purifying performance of NOx but that of HC, thereby upgrading the exhaust gas purifying performance of the three-way catalyst 301 as a whole.

In this case, too, the macroporous catalyst element 301 a and the microporous catalyst element 301 b do not have to be coupled and integrated with each other. On the contrary, the macroporous catalyst element 301 a and the microporous catalyst element 301 b may be located away from each other in the direction of the exhaust flow as another embodiment, as illustrated in FIG. 8.

Next, a third embodiment will be described.

The third embodiment differs from the first simply in that a three-way catalyst 302 is employed in place of the three-way catalyst 30.

FIG. 9 shows a quarter portion of a unit cell of the three-way catalyst 302. The three-way catalyst 302 is formed in layers of a microporous catalyst element 302 a having pores whose average pore opening size is smaller than the molecular size of HC and a macroporous catalyst element 302 b having pores whose average pore opening size is larger than the molecular size of HC. The microporous catalyst element 302 a is disposed on a surface layer side, whereas the macroporous catalyst element 302 b on an internal layer side.

Hereinafter, operation of the exhaust emission control device, in which the microporous catalyst element 302 a is disposed on the surface layer side, and the macroporous catalyst element 302 b on the internal layer side, will be described.

When the rich operation is performed in the O₂ F/B control, and the reducing atmosphere is produced in the three-way catalyst 302, the trapped NOx and O₂ are released to cause the oxidation-reduction reaction with CO and HC contained in the exhaust emission as seen in the above embodiments. In this procedure, since the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed, the released NOx is preferentially reacted with CO, while the released O₂ is well reacted with HC.

Once NOx and O₂ are sufficiently released, CO and H₂ having smaller molecular size than HC are satisfactorily trapped in micropores S of the microporous catalyst element 302 a located on the surface layer side, and HC of large molecular size passes through gaps in the microporous catalyst element 302 a to be successfully trapped in macropores L of the macroporous catalyst element 302 b located on the internal layer side. Like the above embodiments, in the three-way catalyst 302, CO and HC are actively separated from each other and trapped in the microporous catalyst element 302 a and the macroporous catalyst element 302 b, respectively.

When the lean operation is carried out, and the oxidizing atmosphere is created in the three-way catalyst 302, the trapped CO, H₂ and HC are released to produce the oxidation-reduction reaction with O₂ and NOx contained in the exhaust emission. In this case, CO and HC are separately trapped in the microporous catalyst element 302 a and the macroporous catalyst element 302 b, respectively, as mentioned above. Also CO and H₂ are released in the microporous catalyst element 302 a located on the surface layer side. Therefore, the released CO and H₂ are preferentially and surely reacted with NOx contained in the exhaust emission in the microporous catalyst element 302 a located on the surface layer side, due in part to the fact that the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed. By using CO for reaction with NOx in this manner, the released HC is fully reacted with O₂ contained in the exhaust emission in the macroporous catalyst element 302 b located on the internal layer side.

This upgrades not only the purifying performance of NOx but that of HC, thereby enhancing the exhaust gas purifying performance of the three-way catalyst 302 as a whole.

Furthermore, if the microporous catalyst element 302 a and the macroporous catalyst element 302 b are formed in layers, at the time of cold start of the engine 1, the microporous catalyst element 302 a and the macroporous catalyst element 302 b are raised in temperature substantially at the same time and are successfully activated.

A fourth embodiment will be described below.

The fourth embodiment differs from the third only in that a three-way catalyst 303 is employed in place of the three-way catalyst 302.

FIG. 10 shows a quarter portion of a unit cell of the three-way catalyst 303. The three-way catalyst 303 is formed in layers of a macroporous catalyst element 303 a having pores whose average pore opening size is larger than the molecular size of HC and a microporous catalyst element 303 b having pores whose average pore opening size is smaller than the molecular size of HC. The macroporous catalyst element 303 a is disposed on the surface layer side, and the microporous catalyst element 303 b on the internal layer side. In other words, in the three-way catalyst 303, the microporous and macroporous catalyst elements are reversely positioned, compared to the three-way catalyst 302.

Hereinafter, operation of the exhaust emission control device, in which the macroporous catalyst element 303 a is disposed on the surface layer side, and the microporous catalyst element 303 b on the internal layer side, will be described.

When the rich operation is implemented in the O₂ F/B control, and the reducing atmosphere is generated in the three-way catalyst 303, the trapped NOx and O₂ are released to cause the oxidation-reduction reaction with CO and HC contained in the exhaust emission as seen in the above embodiments. Since the oxidation-reduction reaction of CO and NOx is faster than that of HC and NOx in reaction speed, the released NOx is reacted with CO by priority, whereas the released O₂ is well reacted with HC.

Once NOx and O₂ are sufficiently released, HC of large molecular size is satisfactorily trapped in macropores L of the macroporous catalyst element 303 a located on the surface layer side. On the other hand, CO having smaller molecular size than HC is successfully trapped in micropores S of the microporous catalyst element 303 b located on the internal layer side. Like the above embodiments, in the three-way catalyst 303, HC and CO are actively separated from each other and trapped in the macroporous catalyst element 303 a and the microporous catalyst element 303 b, respectively.

When the lean operation is performed, and the oxidizing atmosphere is created in the three-way catalyst 303, the trapped HC and CO are released to produce the oxidation-reduction reaction with O₂ and NOx contained in the exhaust emission. In this case, as mentioned above, HC and CO are trapped separately in the macroporous catalyst element 303 a and the microporous catalyst element 303 b, respectively. Therefore, the released HC is well reacted with O₂ contained in the exhaust emission in the macroporous catalyst element 303 a located on the surface layer side, and the released CO is relatively well reacted with NOx contained in the exhaust emission without being severely hindered by the released HC in the microporous catalyst element 303 b located on the internal layer side.

Consequently, not only the purifying performance of NOx but that of HC is upgraded, which enhances the exhaust gas purifying performance of the three-way catalyst 303 as a whole.

In this case, too, the macroporous catalyst element 303 a and the microporous catalyst element 303 b are formed in layers, so that at the time of cold start of the engine 1, the macroporous catalyst element 303 a and the microporous catalyst element 303 b are raised in temperature substantially at the same time and are successfully activated.

A fifth embodiment will be described below.

The fifth embodiment is different from the first simply in that A/F modulation (air-fuel ratio modulating means) is forcibly implemented instead of performing the O₂ F/B control.

FIG. 11 shows a control routine of A/F modulation control in a flowchart. Explanations will be provided with reference to the flowchart.

First, Step S20 judges whether a time counter has counted predetermined time t1. The predetermined time t1 is set equal to or less than the amount of time that is expected to be taken before a trapping amount of CO in the microporous catalyst element 30 a of the three-way catalyst 30 reaches a saturated state, or a breakthrough point, based on for example a preliminary experiment or the like. That is to say, Step S20 judges whether the trapping amount of CO is at a stage immediately before reaching the breakthrough point.

If a result of the judgement in Step S20 is “NO”, which means that the prescribed time t1 has not yet elapsed, the trapping of CO is considered to be quite possible. The procedure then advances to Step S22, and the rich operation is carried out or continued. On the contrary, if the result of the judgement is “YES”, which means that the prescribed time t1 has elapsed, the procedure proceeds to Step S24.

In Step S24, it is judged whether the time counter has counted prescribed time t2. Prescribed time t2-t1 is set equal to or less than time that is expected to be taken before a trapping amount of NOx in the microporous catalyst element 30 a of the tree-way catalyst 30 reaches a saturated state, or a breakthrough point, based on for example a preliminary experiment or the like. In short, Step S24 judges whether the trapping amount of NOx is at a stage immediately before reaching the breakthrough point.

If a result of the judgement of Step S24 is “NO”, which means that the prescribed time t2 has not yet elapsed, NOx is considered to be quite trappable. Subsequently the procedure advances to Step S26, and the lean operation is implemented or continued. On the contrary, the result of the judgement of Step S24 is “YES”, which means that the prescribed time t2 has elapsed, the procedure proceeds to Step S28, and the time counter is reset to zero. Thereafter the rich operation and the lean operation are repeatedly implemented.

Basically in the fifth embodiment, the exhaust A/F is efficiently modulated between the rich A/F and the lean A/F within a range in which the trapping amounts of CO and NOx in the three-way catalyst 30 do not reach the respective breakthrough points.

As a consequence, in the exhaust emission control device according to the fifth embodiment, the exhaust A/F is efficiently modulated between the lean A/F and the rich A/F due to the A/F modulation control, to thereby satisfactorily create the oxidizing and reducing atmospheres. Therefore, CO and HC repeatedly and satisfactorily continues to be trapped in the three-way catalyst 30 while being separated from each other in the reducing atmosphere. In the oxidizing atmosphere, the released CO is surely reacted with NOx contained in the exhaust emission by priority without being hampered by the released HC. Accordingly, the purifying performance of NOx is improved. By using CO for reaction with NOx, the released HC is fully reacted with O₂ contained in the exhaust emission, resulting in the enhancement of the purifying performance of HC. Consequently, the exhaust gas purifying performance of the three-way catalyst 30 is upgraded as a whole and is constantly maintained in a high level.

Although the three-way catalyst 30 of the first embodiment used in the above explanation, the three-way catalyst is not limited to this, and the fifth embodiment is applicable if using any one of the three-way catalysts 301, 302 and 303 of the second, third and fourth embodiments.

In the above-described embodiments, the three-way catalyst is provided with the microporous and macroporous catalyst elements that are different in average pore opening size, to thereby separate CO and HC, that is, two components contained in the exhaust emission. It is also possible, however, to further vary the average pore opening size according to components to be trapped and dispose three or more catalyst elements (pore group), to thereby separate three or more components contained in the exhaust emission. Furthermore, components to be separated, which are contained in the exhaust emission, are not limited to CO and HC. On the contrary, the components to be separated may be selected as needed.

Although in the above embodiments, the gasoline engine is employed as the engine 1, the engine 1 may be a diesel engine.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. An exhaust emission control device for an internal combustion engine, comprising: a three-way catalyst located in an exhaust channel of an internal combustion engine, wherein: said three-way catalyst comprises one or more catalyst elements and has two or more pore groups different in average pore opening size in a washcoat.
 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein: said three-way catalyst comprises a microporous catalyst element having a micropore group whose average pore opening size is smaller than prescribed size in a washcoat and a macroporous catalyst element having a macropore group whose average pore opening size is larger than said prescribed size in a washcoat.
 3. The exhaust emission control device for an internal combustion engine according to claim 2, wherein: said three-way catalyst comprises a microporous catalyst element having a micropore group whose average pore opening size is smaller than molecular size of HC in a washcoat and a macroporous catalyst element having a macropore group whose average pore opening size is larger than the molecular size of HC in a washcoat.
 4. The exhaust emission control device for an internal combustion engine according to claim 2, wherein: said microporous catalyst element and said macroporous catalyst element are arranged in series with each other, facing in a direction of an exhaust flow.
 5. The exhaust emission control device for an internal combustion engine according to claim 4, wherein: said microporous catalyst element is disposed on the upstream side of the exhaust flow, and said macroporous catalyst element on the downstream side of the exhaust flow.
 6. The exhaust emission control device for an internal combustion engine according to claim 2, wherein: said microporous catalyst element and said macroporous catalyst element are arranged in layers.
 7. The exhaust emission control device for an internal combustion engine according to claim 6, wherein: said microporous catalyst element is disposed on a surface layer side, and said macroporous catalyst element on an internal layer side.
 8. The exhaust emission control device for an internal combustion engine according to claim 1, further comprising: air-fuel ratio modulating means for periodically modulating an air-fuel ratio of an exhaust emission that flows into said three-way catalyst between a lean air-fuel ratio and a rich air-fuel ratio. 